Such grooved choke plates are used, for example, as the time adjustment element in an adjustable choke which is for example part of a pneumatic RC-timer (see in this respect German offenlegungsschrift specification 2,204,667 for example). The upper and lower limits for the adjustment of the time range of such a timer may, in commonly used systems, be different by a factor of 100, such chokes having to be so designed so that precise adjustment of times within the full adjustment range may be produced and kept to.
Because the resistance to flow of the choke duct, in the form of an open choke groove shut off by a cover plate thereon, is dependent on the fourth power of the depth of the choke groove, very tight conditions have to be kept to with respect to the accuracy of making such systems, and in fact such conditions may generally in the prior art only be kept to by high-price, separate machining of the plates.
If, for reasons of price, there is to be no such highly accurate machining, choke adjustment, to be precise, and to get to a given time of such a timer using a grooved choke plate, is only possible on later use of the choke by using a stopwatch, because it is not possible to undertake such adjustment simply on the basis of the adjustment scale on the choke. For this reason, any lower price possible on mass producing the grooved choke plate will be outbalanced by the loss of time on adjustment of the choke on later operation, which is likely to be of special weight if frequent changes are to be made in the adjustment of the choke, as will probably be the case on changing pneumatic program control systems using pneumatic timers.